Crucial 4 GB DDR3 Desktop RAM, 1333MHz, SODIMM, 1.35V

In short, 204-pin small outline dual inline memory modules (SODIMMs) are the form factor used to provide DDR3 memory modules for notebook computers and other small form factor computing platforms. 204-pin SODIMMs are currently available in PC3-8500 (DDR3 1066MHz) and PC3-10600 (DDR3 1333MHz) speeds

To use DDR3 memory, your system must have both a 204-pin SODIMM slot and a chipset that supports DDR3 memory. A DDR3 SODIMM will not fit into DDR2 or DDR memory sockets. (Information about which memory technology your system uses is included in the Crucial Memory Advisor™ tool.)
SODIMMs are smaller and thinner than regular DIMMs, and the 204-pin SODIMM PCBs are always -2.6 inches long and are usually around 1.18 inches high